Array: 3090T__50_2000_100 Total Volume: 3090.0 cubic inch
20 15 10 5
0 -5
21 22
-10 -15
-20 -5.0 -2.5 0.0 2.5 5.0 7.5 X (metres) b 50 0 10.0 12.5 15.0 17.5
150 150
23 24
150 150
25 70 26 40 27 20
28 29
70 70
30 31
250 250
Inactive guns Single guns Cluster guns
a
1 2
150 150
3 4
60 60
5 20 6 40 7 60
8 9
100
10 11
250 250
3.2.3 Sound Propagation with Horizontal Distance
1 2
100 100
14 50 15 60 16 20 17 40
18 70
10 11
250 250
Te signals from marine airgun arrays can be detected in the water column many kilometres away from the seismic vessel, sometimes 100 km or more. Te sound levels from airguns at long horizontal distances from the seismic vessel are determined not only by the acoustic power output but, equally importantly, by the local sound transmission conditions. Tis is discussed in more detail in Section 3.5. Figure 3.16 shows the sound recorded 13 km away from
-50 0 200 Time (ms) c 200
an airgun array in a water depth of 70m. Te signal has three important features at times T0=7.70s, T1=8.78s and T2=9.77s. Tese are the arrival times of three ‘wavelets’ travelling with apparent velocities c0=1,687 m/s, c1=1,480 m/s and c2=1,330 m/s, respectively. Te low-amplitude, low-frequency wave starting at T0=7.70s is called the ground wave because it is closely related to the sediment sound velocity. Te lowest frequency component arrives first (at T0 with velocity c0), followed by progressively higher frequency components travelling at progressively lower velocities. Te highest frequency component of the ground wave arrives at time T2 with velocity c2. Te high-amplitude, high-frequency wave which is superimposed on the ground wave at time T1=8.78s is called the water wave because it is mainly a function of the water sound velocity. In the water wave, with duration T=1.2s, higher frequencies travel fastest and arrive before lower frequencies. At time T2 the frequencies of the ground wave and
Figure 3.14: In-line (upper panel) and cross-line (lower panel) directivity plots for the airgun array, at 60m depth observation point. Colours indicate different energy levels in dB.
180
0
200
400 Frequency (Hz)
Figure 3.13: (a) Configuration (viewed from top) of an airgun array of total chamber volume 3,090 in3
(50.7 l). (b) Modelled far-field pressure signature of the
sound pulse referred to 1m distance from the source centre. The source depth is 5m and the gun pressure is 2,000 psi. The strength (peak-to-peak amplitude) is 135.6 bar-m. The primary-to-bubble ratio is 35.6. The water velocity is 1,506.9 m/s. (c) Frequency spectrum of the modelled far-field pressure signature.
an acoustic waveguide where the sound propagates with significantly less attenuation than sound in an infinite water pool. Te transmission properties of this waveguide depend on the geology of the seafloor and variation of sound velocity with depth and distance. For a soft seafloor, the critical angle is typically 60–70
degrees; for a hard sea bottom it can become 30 degrees. More sound enters the waveguide for hard seafloors than soft ones, producing a higher level of sound at large range from the source.
101
Amplitude spectrum (dB re 1 µPa/Hz-m)
Acoustic Pressure (bar-m)
Y (metres)
PGS
PGS
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